Fully Parametric Square Hole Box (for nail polish)

Fully parametric version of my older design http://www.thingiverse.com/thing:2033871

Removal of ```fudge``` variable makes it less flexible than v1, but easier to customise - it will always leave the top most row square and not sliced along the diagonal like the lower rows.

```bottom``` should be set to the height of the lowest row, obviously smaller than ```zheight```.

It doesn't have to be square, you can set ```xlength``` to a different value to ```ywidth```.